1. Field of the Invention
The present invention relates to mold clamping systems for injection molding machines and, more particularly, to a structural arrangement for locking a movable platen to a plurality of strain rods during the mold clamping and holding phase of an injection molding cycle.
2. Description of the Related Art
Large capacity, hydraulically-operated injection molding machines of the prior art generally incorporate a mold clamping system that includes three distinct platens, two spaced, opposed, stationary platens and a movable platen positioned between the stationary platens. Four parallel strain rods extend between and connect with each of the stationary platens, and the movable platen is carried on and movable along the strain rods. One of the stationary platens supports one portion of a mold and the movable platen supports a cooperating mold portion that defines a mold cavity when the two mold portions are brought together. The movable platen is usually moved toward and away from the mold-carrying stationary platen by a large diameter hydraulic piston and cylinder that also provides the clamping force to hold the mold portions together during high pressure injection of molten molding material into the mold cavity. The large diameter, long stroke piston requires a large volume of hydraulic fluid to move the movable platen, and the axial length and the weight of the movable platen are quite high, resulting in relatively slow clamping phases of a molding cycle, and a relatively long overall machine length.
In an effort to reduce the size of such machines, and to provide a movable platen having less weight and therefore less inertia, an alternative mold clamping system was developed that typically includes only two primary platens, one stationary platen and one movable platen. To maintain the mold halves in tightly contacting relationship during the mold clamping and injection phases of the molding cycle in such a system, the movable platen must be securely held in place relative to strain rods that connect to the stationary platen.
Several approaches have been proposed for locking a movable platen to the clamping system strain rods in a two-platen mold clamping system. For example, in U.S. Pat. No. 3,669,599, entitled "Hydro-Mechanical Clamp," which issued on Jun. 13, 1972, to T. O. Snider et al., there is disclosed a locking arrangement incorporating pairs of scissor-type blocking arms to engage with and clamp around threaded portions of strain rods for holding the movable platen at a particular position on the strain rods, and for preventing movement of the movable platen away from the stationary platen and along the strain rods during the molding material injection phase of the molding cycle. The Snider et al. structure includes a complicated cam and link arrangement.
Another approach to providing a locking structure for a movable platen is disclosed in U.S. Pat. No. 4,059,383, entitled "Mold Opening And Locking Mechanism In An Injection Molding Machine Or The Like," which issued on Nov. 22, 1997, to Tsutsumi. In that approach an actuator is provided to actuate a locking arrangement to move a plurality of locking plates into and out of engagement with a gap provided between an annular supporting panel and a lock adjusting member.
A further approach to connect a platen to a strain rod is shown in U.S. Pat. No. 3,951,579, entitled "Apparatus for Facilitating Access to the Mold Halves of a Molding Machine," which issued on Apr. 20, 1976, to Myers et al. In the Myers et al. structure an actuating cylinder is provided adjacent one of the strain rods to move clamping arms into and out of engagement with the strain rod to clamp the strain rod to a stationary platen. When the clamping arms are opened, the strain rod can be retracted from the stationary platen to provide larger access to the mold halves.
A still further approach is disclosed in U.S. Pat. No. 5,868,989 entitled "Method For Clamping Platens Of Injection Molding Machines," which issued on Feb. 9, 1999 to Glaesener et al. A locking arrangement is disclosed that includes rotatable, toothed, hollow pistons that include circumferentially spaced, axially aligned internal teeth that are engageable with corresponding teeth formed on the clamping system strain rods. When the respective teeth are engaged, axial movement of a strain rod relative to a piston is prevented.
Although several arrangements have been disclosed for locking a platen relative to strain rods of a mold clamping system, the disclosed arrangements involve either apparatus that requires considerable motion of the locking elements, thereby extending the overall molding cycle operating time, or they involve relatively complex mechanisms that add significantly to the cost of machines that incorporate such arrangements. It is an object of the present invention to overcome the limitations of the heretofore-disclosed devices and to provide a locking arrangement that involves minimal motion, and thereby a shorter cycle time, and that also involves a simpler, less costly construction.